1. Field of the Invention
The present invention relates to amplifier apparatus that has: a non-linear high-frequency power amplifier amplifying a first input signal; and a power supply voltage control section receiving as input an external power supply and forming a power supply voltage for the high-frequency amplifier based on a second input signal, and that amplifies the signal level of the first input signal using the high-frequency power amplifier to a level corresponding to the second input signal, and the present invention furthermore relates to polar modulation transmission apparatus and wireless communication apparatus mounted with this kind of amplifier apparatus.
2. Description of the Related Art
In the related art, there has typically been a trade off relationship between power efficiency and linearity in the design of amplifier apparatus employed as linear transmission modulators. However, recently, amplifier apparatus have been proposed that make possible both high efficiency and linearity in linear transmission modulators by using polar modulation.
FIG. 1 is a block diagram showing an example configuration of amplifier apparatus to which polar modulation is applied. Amplifier apparatus 1 is configured so as to have non-linear high-frequency power amplifier 2 and power supply voltage control section 3.
A baseband amplitude modulation signal (for example, √(I2+Q2)) 4 split from a baseband modulation signal by an amplitude phase splitting section (not shown) is input to power supply voltage control section 3. Power supply voltage control section 3 then forms a power supply voltage for high-frequency power amplifier 102 based on baseband amplitude modulation signal 4. The power supply voltage formed by power supply voltage control section 3 is then supplied to high-frequency power amplifier 2.
A phase-modulated high-frequency signal 5 is input to high-frequency power amplifier 2. The phase-modulated high-frequency signal 5 is obtained by first splitting the phase component of a baseband modulation signal (for example, an angle between a modulation symbol and I axis) using an amplitude phase splitting section (not shown), and then modulating a carrier frequency signal using this phase component.
High-frequency power amplifier 2 is composed of a non-linear amplifier, amplifying a signal resulting from multiplying a power supply voltage value and phase-modulated high-frequency signal 5 by the gain of the high-frequency power amplifier 2, and outputting this as a transmission output signal 6. Transmission output signal 6 is sent from an antenna (not shown).
By using a polar modulation scheme in this way, it is possible to make the phase-modulated high-frequency signal 5 inputted to the high-frequency power amplifier 2 into a constant-envelope signal having no fluctuation component in amplitude directions. It is therefore possible to use a high-efficiency non-linear amplifier as high-frequency power amplifier 2. As a result, the amplifier apparatus 1 of the configuration of FIG. 1 makes possible both high-efficiency and linearity.
However, in order for power efficiency to be a maximum, it is often the case that power supply voltage control section 3 is implemented using a switching mode power supply having a D-class amplifier as an output stage. As cases where a normal switching mode power supply is implemented using pulse width modulation are common, the output of this kind of power supply is such that the ratio of Hi (high level)/Lo (low level) becomes a rectangular wave representing baseband amplitude modulation signal 4.
However, when pulse width modulation is carried out at the power supply voltage control section 3 as described above, intermodulation distortion occurs in the transmission output signal. As technology to resolve this, as shown in FIG. 2, power supply voltage control section 3 has been given a delta modulator configuration composed of an adder 11, quantizer 12, low pass filter 13, compensator 14, and attenuator 15, with the baseband amplitude modulation signal 4 being delta-modulated and supplied to the high-frequency power amplifier 2 (for example, refer to paragraphs 0008-0009 and FIG. 11 of Laid-Open Japanese Patent Application Publication No. Hei. 10-256843). As a result, it is possible to improve the distortion appearing in the transmission output signal 6 by implementing the utilization of delta modulation for the switching mode power supply and using a negative feedback loop for this delta modulation.
However, large current switching is necessary for the quantizer 12 to drive the high-frequency power amplifier 2. Further, when the frequency band of baseband amplitude modulation signal 4 inputted to power supply voltage control section 3 is broad, it is necessary for the sampling rate of quantizer 12 to be a high-speed. Typically, the large current operation and high-speed operation of a switching element has a trade-off relationship, and this makes design of quantizer 12 requiring both a large current and high-speed switching operation extremely difficult.
Therefore, as shown in FIG. 3, there has been proposed a digital modulator configuration where power supply voltage control section 3 employs a polyphase quantizer 21 (for example, refer to paragraph 0010-0012 and FIG. 12 of Laid-Open Japanese Patent Application Publication No. 2001-156554). As shown in FIG. 4, polyphase quantizer 21 is configured with N quantizers (1 to N) 22-1 to 22-N. Each of the quantizers 22-1 to 22-N operates on a phase shift of (360/N) degrees at a speed of (1/N) with respect to the sampling rate in the event of configuring a delta modulator using one quantizer as shown in FIG. 2. Outputs of quantizers 22-1 to 22-N are combined by combiner 23. For ease of description, a description will be given of the case of providing combiner 23 directly after quantizers 22-1 to 22-N, but it is also possible to provide a low pass filter between the quantizers 22-1 to 22-N and the combiner 23 and combine after passing outputs of the quantizers 22-1 to 22-N through the low pass filter.
FIG. 5 shows waveforms for the operation (in the case of N=4) of the polyphase quantizer 21. The final output waveform of polyphase quantizer 21 is of the form shown in FIG. 5A, and combined waveforms of the outputs of the plurality of quantizers 22-1 to 22-N are as shown in FIG. 5B to FIG. 5E. By using this kind of polyphase quantizer 21, it is possible to reduce the speed of quantizers 22-1 to 22-N so as to moderate the requirements on the quantizers 22-1 to 22-N, and amplitude modulation of a broader bandwidth is possible with power supply voltage control section 3.
Unfortunately, when a polyphase quantizer is used, the multiphase configuration requires a greater number of quantizers, and this brings about a corresponding increase in circuit scale. Further, in the event that the quantizer is configured with an analog circuit, characteristic deterioration occurs as a result of variation in characteristics between a plurality of quantizers.